The present invention relates generally to solar or other radiation energy collector/concentrator systems, and more particularly to a focus control system for use with a stretched-membrane mirror module providing for the accurate and rapid focusing or defocusing of incident radiation.
Because of the virtually unlimited supply of energy of the sun and its availability, the conversion of solar radiation into other, more usable forms of energy has long been the subject of serious study and analysis. However, because solar energy is so diffuse, it must first be collected before it can be converted to more usable forms of energy. One technique for collecting solar energy involves a solar central receiver system. Such a system utilizes a large field of mirrors to reflect the sun's energy onto a receiver that is placed at an accessible location, such as on a tower proximate the field of mirrors. The receiver is illuminated with concentrated sunlight to a high temperature, e.g. 565.degree. C., and steam is produced to drive a turbine generator to make electricity. In addition to generating heat for the purpose of generating electricity, the concentrated solar energy can also be used to detoxify waste, or to generate electricity directly with the use of a photovoltaic receiver panel.
The mirrors used in a central receiver system are mounted on a special fixture that directs the reflected image of the sun to the receiver location. Such a mirror/fixture combination is referred to as a heliostat, where the term "helio-stat" means "sun-constant." It is thus the function of the heliostat to position the sun's reflected image on the receiver as the sun moves throughout the day, which receiver is at a fixed location relative to the heliostat.
Early heliostats used glass or metal mirrors that were either small and flat, or slightly concave, and mounted on a tracking mechanism designed to maintain the sun's reflected image on the receiving tower. Because such tracking systems are expensive, it was evident that larger reflector surfaces should be employed in order to collect more energy per each tracking system used. Hence, the heliostat size increased by adding more and larger mirror facets, and truss support structure became heavy in order to limit gravity and wind load deflections. Unfortunately, the increased weight associated with these larger heliostats introduced new problems that had to be addressed if such systems were to be efficiently and cost effectively fabricated and operated. In particular, the size of the support structure needed to maintain the heavier mirror surfaces at a desired orientation, particularly in the presence of other environmental forces, such as wind, rendered very large mirror surfaces impractical. Further, the response time of such metal/glass heliostats, i.e., the time required to remove the sun's image from the receiver location, became unacceptably long, because the tracking system had to move the mirror surface so the sun's image would miss the receiver tower.
A significant advance in the heliostat art occurred with the development of light-weight heliostats employing reflective surfaces mounted on stretched membranes, as taught for example in U.S. Pat. No. 4,511,215. The stretched-membrane concept was radically different from the glass/metal designs in that the mirror module consisted of two thin membranes (metal preferred) stretched over either side of a large-diameter ring (metal preferred). The reflective surface was laminated onto the front membrane. A force was applied to the front membrane to cause focusing by mechanical means or differential pressure. The pressure in the space between the two membranes, referred to as the plenum pressure, was actively controlled by inflating it, using blowers and/or pumps and valves, to provide a concave or parabolic shape to the reflective surface for focusing. The plenum pressure could also be changed through inflation to defocus the mirror for safety procedures.
Unfortunately, the light-weight stretched-membrane mirror module is subject to fluctuations in position from the effects of blowing and gusting winds. Therefore, the focus control system used with such lightweight mirror modules must not only initially set the proper focal length for the mirror by setting the pressure differential across it, but must also compensate for external pressure changes on the front membrane by adjusting the internal plenum pressure, to keep the pressure differential constant.
The known focus systems used with prior stretched-membrane mirror modules consist of a method of plenum evacuation in which the mass of air located in the plenum is increased or decreased to focus or defocus the mirror. Such plenum evacuation is performed using a pump or fan to move large volumes of air. Disadvantageously, the response time of such evacuation focus control systems remains unacceptably long compared to the sudden gusts or changes of wind that may occur, or compared to the rapid defocus time that may be needed for safety reasons. What is needed therefore, is a focus control system for a stretched-membrane mirror module system that can rapidly and accurately change the plenum pressure so as to achieve a desired change in the focal length of the mirror. The present invention advantageously addresses this and other needs.